The broad objective of the proposed research is to determine how closely the integrated EMG recorded from surface electrodes reflects the total muscle fiber activity occurring during different types of human voluntary contractions, in various muscles having different anatomical and physical properties. The EMG/force relationship will be studied for each muscle or group of muscles, during isometric contractions and when shortening or being stretched at a number of controlled velocities. The activity of synergists and antagonists will also be investigated. Computer analyses will be made to study the effect on the surface EMG of activity in fibers of different size and at different distances from the surface. The results will be analyzed to determine whether integrated EMG recordings can be used as a means of measuring the force/velocity relationship of intact human muscles and how this may change with the intensity of contraction in heterogenous muscles if larger faster fibers are recruited sequentially. Additional investigations on the site of fatigue in sustained contractions will also be carried out.